《Table 2–Genome editing to enhance quantity, quality, and shelf life.》

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《Genome editing opens a new era of genetic improvement in polyploid crops》

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MNs or homing endonucleases（HEs）are highly specific endonucleases with large cleavage sites（>14–40 bp，Fig.1-A）that can induce homologous recombination in cultured cells，including mammalian cells and plants.They are the first reported SSNs used to produce targeted DSBs at specific loci in eukaryotic genomes.The DSBs produced by MNs can be repaired by nonhomologous end joining（NHEJ）or homology-directed repair（HDR）[38].The engineering of MNs is challenging，because their DNA recognition and cleavage functions are intertwined in a single protein domain（Fig.1-A）[8，40].On the basis of sequence and structural motifs，homing endonucleases are divided into five families:GIY-YIG，HNH，His-Cys box，PD-（D/E）XK，and LAGLIDADG[41].Homing endonucleases can tolerate target-site polymorphism without loss of binding and cleavage activity.A long exact target site occurs rarely in the genome.For example，only one I-Sce I target site is found in the 13 Mb yeast genome[40].Meganucleases technology has worked well with I-Sce I-mediated recombination in mice，bacteria，mosquitoes，plants，and flies.This technology serves as a genome-editing tool（Tables 1）that allows targeting of chromosomal locus in various genomes[42–46].But owing to the target-sequence specificity of meganucleases，the technology is not commonly used.